Conventionally, a control device that prohibits slip control of a torque converter via slip engagement of a lockup clutch during a period from the time an estimate of a clutch facing temperature becomes equal to or higher than a slip control prohibition temperature to the time when the estimate of the clutch facing temperature becomes equal to or lower than a slip control permission temperature in order to prevent overheat is known as this type of control device for a lockup clutch (see, e.g., Patent Document 1). While the slip control is being prohibited, this control device causes the torque converter to function in a lockup state where front-rear differential rotation, namely a slip rotational speed, of the lockup clutch has a value of 0. This control device thus suppresses reduction in durability of the lockup clutch due to early wear of clutch facing.